The present invention relates to an improved process for adiabatic, non-oxidative dehydrogenation of hydrocarbons whereby a hydrocarbon feed stream is passed through a catalyst bed containing at least a first layer of a first catalyst and a second layer of a second catalyst, wherein the catalyst of the first layer exhibits high activity, but also a higher capacity to produce coke than the second catalyst, while the catalyst of the second layer also exhibits high activity but a reduced capacity to make coke. Various preparation and/or treatment processes and/or differences in the composition of the catalysts contained in the two layers can cause the differences in capacity of the catalysts to make coke.
Alkane dehydrogenation is a recognized process for the production of a variety of useful hydrocarbon products, such as isobutylene for conversion to MTBE, isooctane and alkylates to supplement and enrich gasolines and propylene for use in the polymer industry. There are several processes recognized for the catalytic dehydrogenation of light alkanes, including the Süd-Chemie HOUDRY® process. The catalysts that are used in these dehydrogenation processes may be manufactured from different materials. For example, the HOUDRY® process normally utilizes chromia-alumina catalysts. While not limited, the process of the present invention is especially designed for use with the HOUDRY® dehydrogenation process.
In the HOUDRY® process, an aliphatic hydrocarbon, such as propane, is passed through a dehydrogenation catalyst bed, which may contain various layers of catalysts, where the hydrocarbon travels from one layer to the next and in the process is dehydrogenated to its complimentary olefin. Because the dehydrogenation reaction is endothermic and the process is adiabatic, the temperature of the catalyst bed decreases during the dehydrogenation cycle. At the same time, paraffin conversion declines until conversion is no longer economical. The hydrocarbon flow is stopped at this point. After a steam purge, the catalyst is subjected to a regeneration cycle in air in order to remove coke that has been deposited on the catalyst. During the regeneration cycle the catalyst bed gains heat, some of which is produced by burning of the coke. Thus, coke combustion plays an important role in the heat balance in the catalyst bed. After regeneration the catalyst is reduced, and the cycle is repeated. This process is discussed in detail, for example in U.S. Pat. Nos. 2,419,997 and 5,510,557 and U.S. patent application Ser. No. 20040087825, which references are incorporated herein by reference.
Due to equilibrium limitations, dehydrogenation processes require relatively high operating temperatures. However, as the temperature is increased, a point is reached where the production of undesirable by-products, such as light gas and coke, is so high that the yield of the desired olefin begins to decline.
Thus, it would be advantageous if a method could be developed for the dehydrogenation of aliphatic hydrocarbons that improves olefin selectivity and yield by optimizing the performance of the catalysts of the catalyst bed, especially for production of coke, by various methods such as by adjusting the composition and/or performance of the catalysts contained in the catalyst bed.